Operando Fe dissolution in Fe–N–C electrocatalysts during acidic oxygen reduction: impact of local pH change

Atomic Fe in N-doped C (Fe–N–C) catalysts provide the most promising non-precious metal O2 reduction activity at the cathodes of proton exchange membrane fuel cells. However, one of the biggest remaining challenges to address towards their implementation in fuel cells is their limited durability. Fe demetallation has been suggested as the primary initial degradation mechanism. However, the fate of Fe under different operating conditions varies. Here, we monitor operando Fe dissolution of a highly porous and >50% FeNx electrochemical utilization Fe–N–C catalyst in 0.1 M HClO4, under O2 and Ar at different temperatures, in both flow cell and gas diffusion electrode (GDE) half-cell coupled to inductively coupled plasma mass spectrometry (ICP-MS). By combining these results with pre- and post-mortem analyses, we demonstrate that in the absence of oxygen, Fe cations diffuse away within the liquid phase. Conversely, at −15 mA cm−2geo and more negative O2 reduction currents, the Fe cations reprecipitate as Fe-oxides. We support our conclusions with a microkinetic model, revealing that the local pH in the catalyst layer predominantly accounts for the observed trend. Even at a moderate O2 reduction current density of −15 mA cm−2geo at 25 °C, a significant H+ consumption and therefore pH increase (pH = 8–9) within the bulk Fe–N–C layer facilitate precipitation of Fe cations. This work provides a unified view on the Fe dissolution degradation mechanism for a model Fe–N–C in both high-throughput flow cell and practical operating GDE conditions, underscoring the crucial role of local pH in regulating the stability of the active sites.


Experimental Details 1.1 Synthesis
TAP 900@Fe: 2,4,6-triaminopyrimidine (97%, Sigma Aldrich) and MgCl 2 •6H 2 O (99%, Sigma Aldrich) were ground together with a pestle and mortar in a respective 1:8 weight ratio and then pyrolyzed in a ceramic crucible at 5 °C min -1 up to 900 °C and then one hour hold, all under 300 mL min -1 flow of N 2 atmosphere (>99.998%,BOC).The resultant materials were ground to fine powder with a pestle and mortar, and then washed with 2 M HCl (prepared by dilution of fuming 37% HCl, Merck) overnight.Next, the solution was filtered, rinsed thoroughly with distilled water, and dried at 80 o C overnight, forming TAP 900.2][3] 60 mg of TAP 900 was placed in a 250 mL round-bottom flask along with 75 mL MeOH (AnalaR NORMAPUR Reag.Ph.Eur., ACS, VWR) under vigorous stirring until a homogeneous dispersion was observed.Then, 75 mL of MeOH solution containing 25×10 -3 M FeCl 2 (98% Sigma Aldrich) was added, and the solution was refluxed at 85-90 °C for 24 h.The product was then filtered and rinsed with MeOH, and the powder washed with 0.5 M H 2 SO 4 (95-98% Sigma Aldrich) overnight.Finally, the obtained material was filtered and rinsed abundantly with distilled water and dried at 80 °C to obtain TAP 900@Fe.
FeNC samples and their electrodes were stored under O 2 -free environments (vacuum or N 2 /Ar boxes) to prevent spontaneous aerobic ageing of the FeNC. 4

Operando Characterisation
Flow Cell and Rotating Disc Electrode Preparation: consisted of a 5 mm glassy carbon disc (SIGRADUR, HTW GmbH) in a homemade PEEK shroud.RDE tips were cleaned in Caro acid (prepared by mixing in 1:1 ratio of 30% H 2 O 2 with 96% H 2 SO 4 ) for at least 5 hours.The RDE tips were then boiled in Milli-Q water twice and placed in an oven at 60 o C to remove remaining water.The RDE tip was then polished with 3 µm and then 1 µm diamond paste (Presi) on a polishing pad (Presi) to a mirror finish.

RDE Electrode Manufacture:
Catalyst inks for RDE and flow cell comprised of 4 mg FeNC mL -1 , consisting of a ratio of 4 mg TAP 900@ 57 Fe, 480 µL of 18.2 MΩ cm -1 deionized (Milli-Q) H 2 O, 480 µL of isopropanol (Fisher, 99.5 %) and 40 µL of 5 wt.%Nafion ® (5% w/w in water and 1propanol, Sigma Aldrich).The ink was bath sonicated (Fisherbrand FB 15046, 30 W, 50/60 Hz) for 30 mins and left overnight.The ink was bath sonicated again for 1 min prior to drop casting.Inks were drop-cast onto the glassy carbon surface to the desired loading and subsequently dried under rotation at 100-200 rpm with a heat gun parallel to the RDE tip.
Due to the hydrophobicity of the catalyst, a droplet of electrolyte was placed over the RDE tip and placed in a desiccator and pumped down to vacuum.Upon removal, the catalyst was wet, and all visible air bubbles were removed.With the droplet remaining on the catalyst, the RDE tip was placed in the homemade flow cell (Figure S1) or RDE cell.
Flow Cell ICP-MS Protocol: All measurements were conducted at room temperature (21 ± 1 o C).Ar or O 2 was purged in the feed electrolyte (0.1 M HClO 4 or 0.05 M H 2 SO 4 ) for at least 20 mins prior to measurements.The potential was held at 0.9 V RHE for 200 s before and after each measurement to confirm stable background ICP-MS measurement.Ohmic drop (R) in the cell was first determined via electrochemical impedance measurement at open circuit potential (OCP) from the intercept of the real impedance axis.R varied from 5-25 Ω, if R >25 Ω the cell was reinstalled following checks for oxygen bubbles on the catalytic layer surface.Initial 50 cyclic voltammetry (CV) under Ar-saturation between 0.925-0.200V RHE at 50 mV s -1 , followed by 6 CVs under Ar-saturation 0.925-0.200V RHE at 10 mV s -1 and 6 CVs under O 2saturation between 0.925-0.200V RHE at 10 mV s -1 .Next, either accelerated stress testing (AST) under O 2 -saturation was carried out for 1 h, holding the potential at 0.9 and 0.6 V RHE for 3 s intervals, or chronoamperometry (CA) holding at 0.2 or 0.6 V RHE for 1 h under O 2 -saturation.The square wave AST and its potentials mimics the protocol recently established for Fe-N-Cbased PEMFCs. 5For Ar AST, O 2 -saturated measurements were not conducted beforehand due to some O 2 remaining in the system, which may have affected the results.Finally, 6 CVs under O 2 -saturation between 0.925-0.200V RHE at 10 mV s -1 .A synchronization step from 0.9 to 0.2 to 0.9 V RHE was applied at the end of each measurement to align ICP-MS and electrochemical data.The protocol is summarized in Figure S2.In the instance where slow (1 mV s -1 ) CVs were carried out, this proceeded the 6 CVs (in Ar and O 2 ) and was not followed by any subsequent measurement.
Flow Cell ICP-MS Operation: Flowrate of electrolyte was monitored each day (varying from 430-480 µL min -1 ).The ICP-MS was calibrated before and after completing the electrochemical measurement protocol.Four-point calibration curves were measured with a blank and five prepared standard solutions of 56 Fe and 24 Mg at 0, 5, 10, 20 and 50 ppb.The calibration factor of 57 Fe was corrected with its natural abundance of 2.12%.Calibration curves were carried out before and after online ICP-MS measurements with correction applied for drift. 57Fe and 24 Mg were analyzed with a PerkinElmer NexION 2000c spectrometer coupled to the home-made electrochemical flow cell (Figure S2) and data was recorded on Syngistix software.

GDE ICP-MS Protocol:
Each measurement began with 50 cycles at 50 mV s -1 in Ar atmosphere between 0.20 and 0.925 V RHE, with the electrolyte subsequently refreshed (Figure S3).Then, two different electrochemical techniques were used, depending on the gas atmosphere studied.Potentiostatic and galvanostatic techniques were used in Ar and O 2 atmosphere, respectively.This is due to unpredictable uncompensated resistance recorded at varied current densities in O 2 atmosphere during potentiostatic holds and the iR-compensated potential would not be comparable in the two gas conditions.To mimic recently standardized PEMFC AST conditions, 5 under O 2 conditions in GDE, the current was held for 3 s intervals 200 times at -0.05 and -50 mA cm -2 geo , corresponding to ca. 0.85 and 0.6 V RHE, iR-free , respectively.Under Ar, the potential was held for 3 s intervals 200 times between 0.9 and 0.6 V RHE, iR-free .Before and after AST, 240 s of potentiostatic and galvanostatic holds were applied between (0.85 and 0.50 V RHE, iR-free ) and (-1 and -100 mA cm -2 geo ), respectively, as illustrated in Figure S3.

GDE ICP-MS Operation:
An ICP-MS (Perkin Elmer, NexION 350) was operated in dynamic reaction cell mode with methane (99.9995%,Air Liquide) to limit the impact of polyatomic interference between 56 Fe and 40 Ar 16 O + .2.5 ppb of germanium (Ge) in 1 wt.% HNO 3 (Ultrapure, Merck) was used as internal standard to track and compensate for instrument drift during measurements.Five-point calibration lines were measured with a blank and four daily prepared standard solutions of Fe (0, 1, 5, 25 and 50 ppb).Between 0.203-0.226mL min -1 of the electrolyte was continuously extracted via a capillary to the ICP-MS.The collection efficiency (CE) is calculated using Equation S2. (Figure S3).Calculated Fe dissolution (Equation S3) is based on catalyst loadings, flowrate, and collection efficiencies in Table S1.Fe dissolution data from GDE-ICP-MS was smoothed by adjacent-averaging 30 neighboring points.Raman spectra were measured with an inVia Renishaw confocal Raman microscope with a 532 nm incident laser beam focused through a 50× objective (Leica).The laser intensity was minimized to 2.6 mW to avoid laser damage to the samples.

High-Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM)
was performed with a Talos F200i (ThermoFisher Scientific) operated at an acceleration voltage of 200 kV, a beam current of ca 40 pA, and a convergence angle of 10.5 mrad.STEM combined with electron energy loss spectra (EELS) were performed using a C 3 /C 5 corrected Nion microscope USTEM-200.The experiments are done at 60 keV (to limit beam damage) with a convergence semi-angle of 33mrad.HAADF images were acquired with 80-200 mrad collection angle and the EELS were acquired using a MerlinEM direct electron detector (Quantum Detectors Ltd).
Fe Kβ High Energy-Resolution Fluorescence-Detected X-ray Absorption Near Edge Structure (HERFD-XANES) experiments were conducted at beamline ID26, European Synchrotron Radiation Facility (ESRF).The X-ray beam incidence was 45 o , creating a bulk-sensitive XANES signal.The pristine catalyst powder and thin films of catalytic suspensions were analyzed, utilizing three undulators (u35) to produce incoming photons.A pair of cryogenically cooled Si(111) crystals then monochromatized the radiation.To calibrate the incident beam energy, a reference metallic Fe foil set the first inflection point of the Fe K edge at 7112 eV.Note that the XANES spectra of Fe foil was obtained in transmission mode, where the foil was under ultra-high vacuum conditions to preserve its metallic state.Due to the difference in the experimental conditions of Fe foil and the samples discussed here, the direct comparison of the absolute XANES intensities cannot be made.Instead, for Fe foil, focus is made on the shape of the XANES.In the experimental setup, Germanium (Ge) crystal analyzers (Ge(620) for Kβ and Ge(440) for Kα) in Rowland geometry were employed.The TAP 900@Fe powder was blended with BN3 (Aldrich®) at a mass ratio of 1:3 and subsequently compressed into pellets using a 1-ton force.This identical procedure was applied to both FeO (Aldrich®) and Fe 2 O 3 (Aldrich®).These pellets, along with GDE and glassy carbon rods carrying fresh ink or post-mortem catalyst samples were positioned in a sample holder.
X-Ray Diffraction (XRD) patterns were obtained using a PANanalytical X'PERT PRO powder Xray diffractometer with a Cu Kα source operated at 40 kV and 40 mA, with 0.033 o scan step size.
X-ray Photoelectron Spectroscopy (XPS) was measured with a Thermo Fisher K-alpha XPS system, and the spectra analyzed with Avantage software.All spectra were calibrated relative to the carbon C1s peak at 284.8 eV to correct for charging effects.200 scans were measured in N and O spectra.The Fe peak of TAP 900@Fe mixed with Nafion could not be resolved in XPS due to attenuation of the Fe2p signal by the fluorine Auger peak. 6It is also noted that X-rays cause beam damage to Nafion, however since equivalent number of cycles were repeated on XPS peaks (e.g.O1s) for GDE samples any damage will be equivalent for all samples. 7

Equations
The mass activity, m act , can be found via:

Eq. S1
Where L FeNC is the mass loading of catalyst (mg Fe-N-C cm -2 geo ) and j is the current density (mA cm - 2 geo ) at a specified potential.

Eq. S2
Where m Fe,ICP-MS is the total Fe amount collected in the ICP-MS, m Fe,bulk,start and m Fe,bulk,end are the quantity of Fe in the bulk electrolyte before and after the measurement, respectively.The CE values obtained are presented in Table S1.
The rate of Fe dissolution is given by Eq.S3: Where dFe/dt is the rate of Fe dissolution normalised to catalyst loading (ng Fe s -1 g FeNC -1 ), [Fe] is the concentration of Fe (ng Fe L -1 ), Q is flowrate (L s -1 ), and A is the area of the RDE tip (0.19635 cm 2 ) or GDE electrode (2.01 cm 2 ).CE is the collection efficiency.CE is assumed 100% in flow cell ICP-MS and is calculated in GDE ICP-MS (see Table S1 for values).

Kinetic Modelling
The concentrations of proton (C H ), Fe (C Fe ) and hydroxide ions (C OH ) as a function to time (t) and space (x) are modelled by solving a system of partial differential equations describing the effect of mass transport and of reactions in the catalyst layer and in the electrolyte.
From the Bruggeman model:

𝜀
Where τ is the tortuosity factor.
The system of partial differential equations is solved for the initial conditions, at t = 0, C Fe = 0 and C H = 10 -1 M, C OH = 10 -13 M. The following boundary conditions are used: At the gas phase/catalyst layer interface at x = 0:

Figure S1 .
Figure S1.Schematic of flow cell setup with RDE disk electrode tip, Ag/AgCl/3.4M Cl - reference electrode and glassy carbon counter electrode coupled to online ICP-MS.

Figure S2 .
Figure S2.Protocol followed during flow cell electrochemical testing.

Figure S3 .
Figure S3.Protocol followed during GDE-ICP-MS testing.Note, the electrolyte in the GDE halfcell was refreshed between the 50 CV in Ar and the initial step and that the online measurements only started after the electrolyte was refreshed.

Figure S5 .
Figure S5.Comparison between TAP 900@Fe and TAP 900@ 57 Fe for cathodic scan in O 2saturated 0.1 M HClO4.a.) Mass activity, with inset showing mass activity at 0.8 V RHE, iR-free b.) Current density.O 2 reduction in 0.1 M HClO 4 at 10 mV s -1 and 1600 rpm.O 2 -Ar correction applied.TAP 900@Fe data re-plotted from previous work. 1Error represents standard deviation from at least four repeat measurements, and is highlighted with red and blue colors.

Figure S6 .
Figure S6.a.) Amount of 57 Fe dissolution with varying catalyst loading over cyclic voltammetry at 50 mV s -1 .Line of best fit is assumed to intercept the axis through the origin.b.) Corresponding percentage of total catalyst 57 Fe dissolved.Error bars represent standard deviation of at least four separate measurements.

Figure S7 .
Figure S7.Flow cell under a.) O 2 -saturated conditions, with O 2 -Ar correction applied.b.) Arsaturated conditions.c.) Charge passed over six CV.d.) 57 Fe concentration normalized to charge passed and e.) 57 Fe mass at varying catalyst loading over six CV.f.) Percentage of 57 Fe detected at different stages of protocol.Error bars represent minimum of four repeat measurements.

Figure S8 .
Figure S8. 57Fe dissolution over different stability tests in flow cell.All tests over 1 h with 0.2 mg FeNC cm -2 geo .

Figure S10 .
Figure S10.Example of the 50 CVs measured at 50 mV s -1 in Ar condition before online GDE-ICP-MS measurement.The Fe redox peak decreases with each cycle.

Figure S11 .
Figure S11.ORR mass activity of GDE at a.) 20 o C O 2 reduction in GDE for pristine TAP 900@Fe, and after O 2 and Ar 20 o C GDE protocols.b.) 75 o C O 2 reduction in GDE for pristine TAP 900@Fe and after O 2 GDE 75 o C GDE protocol.Error represents two repeat measurements.

Figure S12 .
Figure S12.Peak fitting of fresh TAP 900@Fe GDE for a.) C1s and b.) O1s.c.) Total O1s content in at.% for GDE samples.The content is based on counts from only C and O.

Figure S14 .
Figure S14.Raman spectra of pristine TAP 900@Fe on GDE, and post-GDE protocol at 20 o C (Ar and O 2 ) and 75 o C (O 2 ).

Figure
Figure S16.a.) HAADF-STEM of fresh TAP 900@Fe GDE b.) and HAADF-STEM of particle highlighted in white box in a.), with corresponding EDX spectrum image and EELS.HAADF-STEM and corresponding EDX spectrum image and EELS of GDE c.) Post 20 o C Ar protocol.d.) Post 20 o C O 2 protocol.The oxidation state of Fe and Ca may be influenced by beam damage so are not discussed.

Figure S18 .
Figure S18.XRD of pristine TAP 900@Fe, prepared on GDE and post GDE protocols, with peaks of PTFE and graphite labelled.

Figure S19 .
Figure S19.Ex situ XANES a.) Absorption and b.) pre-edge region c.) Normalised and d.) First derivative of TAP 900@Fe powder and GDE ink, post GDE 25 o C Ar and O 2 protocols and reference FeO, Fe 2 O 3 and Fe foil.Due to the difference in the experimental conditions of Fe foil and the other samples, the direct comparison of the absolute XANES intensities cannot be made for Fe foil.Instead, for Fe foil, focus is made on the shape of the XANES.To note in Figure a.) and b.) spectra are normalized by incoming beam and the area below the spectra, while in c.) an additional normalization is applied based on the maximum intensity.

Figure S21 .
Figure S21.Pourbaix diagram of Fe at 25 o C and [Fe] = 10 -8 M. Fe Pourbaix data replotted from Ref.15 The error bar for O 2 represents variation of pH from different ε and k r in Figure5e.

Table of Contents
Ex Situ Characterisation RDE measurements were carried out in a three-electrode setup, employing an AUTOLAB PGSTAT302N in Ar-(99.999%,Messer) or O 2 (99.999%,Messer) saturated 0.1 M HClO 4 electrolyte.Measurements in 0.1 M HClO 4 (Suprapur, Carl Roth) consisted of initial 50 CV in Ar at 50 mV s -1 followed by O 2 saturated conditions at 1,600 rpm and 10 mV s -1 at 25°C.85% iR correction was applied during O 2 reduction measurement.O 2 reduction capacitance correction was applied by subtracting current under equivalent measurement conditions, under Arsaturation. 1.3

Table S1 .
Loading, flow rate and collection efficiency for GDE ICP-MS experiments.

Table S2 .
Relative content in at.% considering only C and O in GDE.